When you're working with strings in PHP you'll need to pay special attention to the formation, using " or '
$string = 'Hello, world!';
$string = "Hello, world!";
Both of these are valid, the following is not:
$string = "Hello, world';
You must also note that ' inside of a literal started with " will not end the string, and vice versa. So when you have a string which contains ', it is generally best practice to use double quotation marks.
$string = "It's ok here";
Escaping the string is also an option
$string = 'It\'s ok here too';
More information on this can be found within the documentation